Memory devices and methods of forming the same

ABSTRACT

Memory devices having a plurality of memory cells, with each memory cell including a phase change material having a laterally constricted portion thereof. The laterally constricted portions of adjacent memory cells are vertically offset and positioned on opposite sides of the memory device. Also disclosed are memory devices having a plurality of memory cells, with each memory cell including first and second electrodes having different widths. Adjacent memory cells have the first and second electrodes offset on vertically opposing sides of the memory device. Methods of forming the memory devices are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/840,839, filed Jul. 21, 2010, pending, which is a continuation ofU.S. patent application Ser. No. 12/195,510, filed Aug. 21, 2008, nowU.S. Pat. No. 7,772,583, issued Aug. 10, 2010, the disclosure of each ofwhich is incorporated by reference herein in its entirety.

TECHNICAL FIELD

Embodiments of the present invention relate to memory devices andmethods of fabricating the same. In particular, the present inventionrelates to memory devices having laterally constricted phase changematerials or electrodes of different widths, wherein narrow portions ofthe lateral constrictions or narrow electrodes are on opposing sides ofthe memory device and to the fabrication of such memory devices.

BACKGROUND

Phase change random access memory (PCRAM) refers to a non-volatilememory device capable of recording and reading data according to anapplied current. In a PCRAM device a volume of phase change material isdeposited between two electrodes to form a single memory cell. Phasechange materials are used in electronic memory applications because oftheir ability to electrically switch between an amorphous andcrystalline state. These materials selectively exhibit more than onevalue of electrical resistivity. For example, when the phase changematerial is in a crystalline state its resistance is low, and when it isin an amorphous state its resistance is high.

In a PCRAM device, a programming current is passed through the phasechange material to induce a phase change. This programming currentgenerates heat as a result of the electrical resistance of the phasechange material. The amount of heat generated is proportional to thecurrent density in a fixed volume of material. As the volume of materialis decreased the programming current required to induce the phase changealso decreases. Furthermore, as the programming current is reduced theamount of heat generated also decreases.

Since each memory cell utilizes a programming current, and there aremillions of memory cells per PCRAM device, a large overall energy inputis required to operate the device. It is desirable to reduce the amountof programming current required to induce the phase change and, in sodoing, reduce the total energy requirements of the device.

Additionally, there is an increasing need to produce ever-smaller memorydevices. As memory devices are condensed, the relative distance betweenneighboring memory cells is lessened, resulting in cells of extremelyclose proximity. It is theorized that cells in such close proximity willbe subject to increased thermal influence from adjacent cells. Thisphenomenon is known as “thermal cross-talk.” Thermal cross-talk occurswhen heat generated in one memory cell, by application of theprogramming current, is thermally conducted to an adjacent memory cell.

Thermal cross-talk is undesirable because it can cause an unwanted phasechange in a memory cell, resulting in corruption of the data storedwithin the memory cell. Transitions between the amorphous andcrystalline states may be initiated by temperature change. If thermalcross-talk is not prevented, it is possible that the phase changematerial of an unselected cell, one to which current is not applied,will be transformed (i.e., inadvertently programmed to an incorrectstate) due to heat transfer from an adjacent cell. It would be desirableto form a device capable of operating with reduced energy draw andnegligible thermal cross-talk despite minimal scale and high celldensity.

U.S. Patent Application Publication No. 2007/0181932 to Happ et al.describes a method of thermally isolating phase change memory cells.Adjacent phase change memory cells are separated from one another byfirst and second insulating materials. The phase change materials in thephase change memory cells have an hourglass or tapered shape.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIGS. 1-5 are cross-sectional views of memory devices in accordance withembodiments of the invention;

FIGS. 6-14 are cross-sectional views illustrating the fabrication of thememory device of FIG. 1;

FIGS. 15-21 are cross-sectional views illustrating the fabrication ofthe memory device of FIG. 2;

FIGS. 22-24 are cross-sectional views illustrating the fabrication ofthe memory device of FIG. 3; and

FIGS. 25 and 26 are cross-sectional views illustrating the fabricationof the memory device of FIG. 4.

DETAILED DESCRIPTION

A plurality of memory cells having an increased distance betweenprogrammable volumes of adjacent memory cells is disclosed. Theplurality of memory cells may be utilized in a memory device, such as aPCRAM device. The PCRAM device may include a phase change material asthe electrically switchable (programmable) material. The phase changematerial may be configured in the PCRAM device as a phase changematerial element. By increasing the distance between adjacentprogrammable volumes, thermal cross-talk between the memory cells isreduced. To increase this distance, the location of the programmablevolumes 2 of the adjacent memory cells 4 on the memory devices 6, 6′,6″, 6′″ are vertically staggered or offset relative to one another, asillustrated in FIGS. 1-4. Each memory cell 4 includes a phase changematerial element 8B disposed between two electrodes, such as between abottom electrode 10 and a top electrode 12. The memory cells 4 arearranged in an array. By way of non-limiting example, the programmablevolume 2 of one memory cell 4 may be proximate the bottom electrode 10while the programmable volume 2 of another, adjacent memory cell 4 maybe proximate the top electrode 12.

In one embodiment, the phase change material element 8B may include alaterally constricted portion 14 thereof, as illustrated in FIGS. 1-3.The phase change material element 8B includes a narrowing orconstriction in the vertical direction of the memory cell 4. Thelaterally constricted portion 14 may be located at an interface of thephase change material element 8B and the bottom electrode 10 or topelectrode 12, as illustrated in FIG. 1, or at a location within thephase change material element 8B, as illustrated in FIGS. 2 and 3. Thelaterally constricted portion 14 may substantially correspond to theprogrammable volume 2 of the memory cell 4. In another embodiment, whichis illustrated in FIG. 4, bottom electrode 52, 54 or top electrode 58,60 may have a smaller width than the width of the phase change materialelement 8B.

As used herein, the term “programmable volume” means and includes aportion of the phase change material at the laterally constrictedportion thereof that electrically switches between an amorphous stateand crystalline state when a current is applied to the phase changematerial. The programmable volume may be in direct or indirect contactwith the bottom or top electrode of the memory cell. If the phase changematerial does not include a laterally constricted portion thereof, theterm “programmable volume” means and includes a portion of the phasechange material in contact with the bottom or top electrode.

As used herein, the terms “bottom,” “top,” “upper,” and “lower” arerelative terms and describe positions in relation to a substrate uponwhich the memory device is formed. The terms “top” or “upper” refer to aposition distant from the substrate, and the terms “bottom” or “lower”refer to a position in close proximity to the substrate. By way ofnon-limiting example, the phrases “bottom electrode” and “top electrode”are relative terms and mean and include the location of a conductivematerial of the electrode with respect to the substrate. “Bottomelectrode” describes an electrode which is proximal to the substrate,while “top electrode” refers to an electrode which is distal to thesubstrate.

As used herein, the phrase “phase change material” means and includes achalcogenide compound that includes a chalcogen ion and anelectropositive element. The chalcogen ion of the phase change materialmay be oxygen (O), sulfur (S), selenium (Se), tellurium (Te), orpolonium (Po). The electropositive element may include, but is notlimited to, nitrogen (N), silicon (Si), nickel (Ni), gallium (Ga),germanium (Ge), arsenic (As), silver (Ag), indium (In), tin (Sn),antimony (Sb), gold (Au), lead (Pb), bismuth (Bi), aluminum (Al),palladium (Pd), cobalt (Co), platinum (Pt), or combinations thereof. Thechalcogenide compound may be a binary, ternary, or quaternary compoundor alloy of these elements. By way of non-limiting example, thechalcogenide compound may include the following combinations ofelements:

-   -   Ge—Te, In—Se, Sb—Te, Sb—Se, Ga—Sb, Ge—Sb, In—Sb, As—Te, Al—Te,        Si—Sb, Ge—Sb—Te, Te—Ge—As, In—Sb—Te, Te—Sn—Se, Ge—Se—Ga,        Bi—Se—Sb, Ga—Se—Te, Sn—Sb—Te, In—Sb—Ge, Ge—Sb—Se, In—Sb—Se,        Te—Ge—Sb—S, Te—Ge—Sn—O, Te—Ge—Sn—Au, Pd—Te—Ge—Sn, In—Se—Ti—Co,        Ge—Sb—Te—Pd, Ge—Sb—Te—Co, Sb—Te—Bi—Se, Ag—In—Sb—Te, Ge—Sb—Se—Te,        Ge—Sn—Sb—Te, Ge—Te—Sn—Ni, Ge—Te—Sn—Pd, or Ge—Te—Sn—Pt.        The stoichiometry of the elements in the above-mentioned        chalcogenide compounds is not limited. As such, any known        stoichiometry of elements in the above-mentioned chalcogenide        compounds may be used. In one embodiment the chalcogenide        compound is a compound of Ge, Sb, and Te (a GST material), such        as Ge₂Sb₂Te₅, Ge₈Sb₃₂Te₅₆ (also known as Ge₁Sb₄Te₇),        Ge₁₄Sb₂₈Te₅₆ (also known as Ge₁Sb₂Te₄), Ge₄₀Sb₉Te₅₁,        Ge₄₄Sb₅Te₅₁, Ge₂₈Sb₂₇Te₄₅, Ge₅₈Sb₁₉Te₂₃, Ge₁₇Sb₂₇Te₅₆, or        Ge₃₀Sb₁₇Te₅₃. In another embodiment, the chalcogenide compound        is Te₈₁Ge₁₅Sb₂S₂ or Sb₂Te₃.

The programmable volumes 2 of the adjacent memory cells 4 are verticallyoffset relative to one another, as shown in FIGS. 1-4. Stated anotherway, programmable volumes 2 of adjacent memory cells 4 are positioned onopposite sides of the memory devices 6, 6′, 6″, 6′″. As shown in furtherdetail in FIG. 5, the memory cells 4 have a bit-to-bit distance (orthermal distance) d, a height h, and a space s. The bit-to-bit distanced is the distance between programmable volumes 2 of adjacent memorycells 4, the height h is the length of the phase change material, andthe space s is the horizontal distance between the centers of adjacentmemory cells 4. As the size of memory devices 6, 6′, 6″, 6′″ continuesto decrease, the height h may range from approximately 20 nm toapproximately 40 nm and the space s may range from approximately 20 nmto approximately 40 nm. By way of non-limiting example, when the space sis approximately 20 nm, the height h is from approximately 20 nm toapproximately 40 nm. The bit-to-bit distance d is calculated as thesquare root of (h²+s²). By way of non-limiting example, ifs is 20 nm andh is 30 nm, d is approximately 36 nm. In contrast, the bit-to-bitdistance d of a conventional memory device, in which the programmablevolumes of adjacent memory cells are on the same vertical plane as oneanother, is equal to the horizontal distance between the programmablevolumes of the adjacent memory cells. In other words, in a conventionalmemory device where s is 20 nm and h is 30 nm, the bit-to-bit distance dis 20 nm.

The embodiments of the present invention may be practiced in conjunctionwith conventional techniques employed in semiconductor fabrication toproduce the desired memory devices 6, 6′, 6″, 6′″. While the followingdescription provides specific details, such as material types, materialdimensions and processing conditions in order to provide a thoroughdescription of embodiments of the present invention, a person ofordinary skill in the art will understand that the embodiments of thepresent invention may be practiced without employing these specificdetails. In addition, the description provided herein does not form acomplete process flow for manufacturing a PCRAM device, and the PCRAMdevice described below does not form a complete semiconductor device.Only those process acts and structures necessary to understand theembodiments of the present invention are described in detail below.Additional acts to form a complete semiconductor device including thePCRAM device may be performed by conventional techniques.

The illustrations presented herein are not meant to be actual views ofany particular PCRAM devices, but are merely idealized representationswhich are employed to describe embodiments of the present invention. Inaddition, the illustrations are not drawn to scale. Elements andfeatures common between figures may retain the same numericaldesignation.

To form the memory devices 6, 6′, 6″, 6′″, the bottom electrode 10 maybe a so-called “plug” in a dielectric material 13, as illustrated inFIGS. 1-4. The dielectric material 13 may be formed from a suitableinsulative or dielectric material, such as an oxide or a nitride. By wayof non-limiting example, the dielectric material 13 may beplasma-enhanced CVD (“PECVD”) SiO_(z), where z is 1 or 2, PECVD siliconnitride, or standard thermal CVD Si₃N₄. The dielectric material 13 andthe bottom electrode 10 may be formed over a substrate (not shown), suchas a conventional silicon substrate or other bulk substrate including alayer of semiconductor material. As used herein, the term “bulksubstrate” includes not only silicon wafers, but alsosilicon-on-insulator (“SOI”) substrates, silicon-on-sapphire (“SOS”)substrates, epitaxial layers of silicon on a base semiconductorfoundation, and other semiconductor or optoelectronic materials, such assilicon-germanium, germanium, gallium arsenide, or indium phosphide. Thesubstrate may be doped or undoped. To form the bottom electrode 10 as aplug, the dielectric material 13 may be patterned to form apertures,which are filled with the conductive material of the bottom electrode10. The conductive material may include, but is not limited to, W, Ni,tantalum nitride (TaN), Pt, tungsten nitride (WN), Au, titanium nitride(TiN), or titanium aluminum nitride (TiAlN). The bottom electrode 10 maybe formed by a conventional deposition technique, such as by chemicalvapor deposition (CVD) or physical vapor deposition (PVD). Portions ofthe conductive material overlying the dielectric material 13 may beremoved, such as by CMP.

Alternatively, the conductive material of the bottom electrode 10 may beformed as a layer or other three-dimensional configuration over thedielectric material 13, as illustrated in FIG. 6. FIG. 6 illustrates theconductive material of the bottom electrode 10 configured as a layer,along with a patterned phase change material element 8A and a patternedmask material 16A. As described in detail below, after forming theconductive material of the top electrode 12 as a layer, the bottomelectrode 10 and the top electrode 12 may be etched in situ byconventional photolithography techniques to form the memory cells 4.While the majority of the drawings herein illustrate forming theconductive material of the bottom electrode 10 as a plug in thedielectric material 13, additional embodiments are contemplated whereinthe conductive material is formed as a layer.

To form the memory device 6 of FIG. 1, the phase change material 8 maybe fanned over and in contact with the dielectric material 13 and thebottom electrode 10, as shown in FIG. 7. The phase change material 8 maybe formed by a conventional technique including, but not limited to, CVDor PVD. However, other deposition techniques known in the art may alsobe used. The phase change material 8 may have a thickness from about 100Angstroms (Å) to about 1000 Å. A mask material 16 may be applied overthe phase change material 8. The mask material 16 may be a conventionalphotoresist material and may be selected by a person of ordinary skillin the art. The mask material 16 may be developed and etched to form adesired pattern, which is transferred to the phase change material 8 toform patterned phase change material 8A and a plurality of trenches 18therein, as shown in FIG. 8. The plurality of trenches 18 are defined bysidewalls of the patterned phase change material 8A. The mask material16 may also be a hard mask formed from a material including, but notlimited to, amorphous carbon or transparent carbon, or silicon oxide.The patterned phase change material 8A may have a height of from about100 Å to about 1000 Å, such as about 600 Å, and a width of from about 50Å to about 500 Å, such as about 250 Å. The patterned phase changematerial 8A may have any geometrical cross-sectional shape, such ascircular, rectangular, or elliptical, for example. The trenches 18 maybe formed by etching the mask material 16 and the phase change material8 using a single etch or multiple etches, such as by etching the maskmaterial 16 and then the phase change material 8. Depending on thematerials used, the mask material 16 and the phase change material 8 maybe etched using a dry etch process, a wet etch process, or combinationsthereof. The etching of the mask material 16 and the phase changematerial 8 may be conducted using conventional etch chemistries, whichare not described in detail herein. The etch chemistry may be selectedby a person of ordinary skill in the art based on the materials used. Byway of non-limiting example, the mask material 16 and the phase changematerial 8 may be etched by reactive-ion etching or plasma etching. Theetchant may be any one of CF₄, CHF₃, CH₂F₂, C₂F₆, CCl₄, Cl₂, or C₄F₈. Inone embodiment the etchant is CF₄.

The patterned mask material 16A may be selectively removed or trimmed,as illustrated in FIG. 9. The vertical thickness and lateral width ofthe trimmed mask material 16B may be reduced relative to its previousthickness and width. The width of the trimmed mask material 16B may beapproximately equal to a desired width of a laterally constricted phasechange material 8B at its narrowest cross-sectional area (see FIG. 10).Trimming the patterned mask material 16A may be accomplished byconventional techniques, such as by an isotropic etch process. Selectionof the isotropic etchant may depend on the material used as the maskmaterial 16 and may be determined by a person of ordinary skill in theart. By way of non-limiting example, the patterned mask material 16A maybe isotropically etched using, for example hydrofluoric acid or CF₄.

The patterned phase change material 8A underlying the trimmed maskmaterial 16B may be anisotropically etched using the trimmed maskmaterial 16B as an etch mask, as shown in FIG. 10. By way ofnon-limiting example, the patterned phase change material 8A may beanisotropically etched using a dry etch. Portions of the patterned phasechange material 8A may be removed to form laterally constricted phasechange material 8B, which has substantially non-vertical or slopingsidewalls. In memory device 6, the laterally constricted phase changematerial 8B may have a substantially tapered or frustoconical shape. Assuch, a first end 19 of the laterally constricted phase change material8B may have a decreased width compared to a second end 21 of thelaterally constricted phase change material 8B.

The trimmed mask material 16B may be removed using a conventionaletchant, which may be selected based on the material used. An insulatingmaterial 20 may be formed conformally over exposed surfaces of thelaterally constricted phase change material 8B, the dielectric material13, and the bottom electrode 10, as shown in FIG. 11. The insulatingmaterial 20 may be any material known in the art to have dielectricproperties and to be capable of being deposited conformally. Theinsulating material 20 may be deposited by any known depositiontechnique, such as atomic layer deposition (“ALD”), which may be usedfor conformal deposition. The insulating material 20 may be, forexample, an ALD oxide, an ALD nitride, or silicon oxynitride. Horizontalportions of the insulating material 20, such as those disposed on anupper surface of the bottom electrode 10 and on an upper surface of thelaterally constricted phase change material 8B, may be removed, as shownin FIG. 12. These horizontal portions may be removed by conventionaltechniques, which are not described in detail herein. Removing thesehorizontal portions of the insulating material 20 may expose an uppersurface of the laterally constricted phase change material 8B and theupper surface of the bottom electrode 10, forming spaces 30 defined bysidewalls of the insulating material 20 and the upper surface of thebottom electrode 10. Since the insulating material 20 is conformallydeposited on the laterally constricted phase change material 8B, thespaces 30 are bounded by non-vertical or sloping sidewalls.

The spaces 30 may be filled with fill phase change material 24, as shownin FIG. 13. The fill phase change material 24 may be one of thematerials previously described and may be the same as or different thanthe material used as phase change material 8. The fill phase changematerial 24 may include different combinations of elements than thephase change material 8. Alternatively, the fill phase change material24 may include the same elements but have a different stoichiometry thanthe phase change material 8. If the same material is used for the phasechange material 8 and the fill phase change material 24, adjacent memorycells may have the same programming current. By utilizing differentmaterials for the phase change material 8 and the fill phase changematerial 24, adjacent memory cells may have different programmingcurrents. Depending on the cross-sectional contact area of the phasechange material element 8B and the fill phase change material 24 withthe electrodes (electrodes 10, 12, 52, 54, 58, or 60), the materialsused for the phase change material 8 and the fill phase change material24 may be selected to provide similar programming currents for adjacentmemory cells. The spaces 30 may be filled with the fill phase changematerial 24 by a conventional technique. Portions of the fill phasechange material 24 overlying the insulating material 20 and thelaterally constricted phase change material 8B may be removed byconventional techniques, such as by chemical mechanical planarization(“CMP”), to expose the fill phase change material 24 and the insulatingmaterial 20. Since the spaces 30 have sloping sidewalls, the fill phasechange material 24 formed in the spaces 30 also has sloping sidewallsand the laterally constricted portion 14.

The top electrode 12 may then be formed overlying the exposed surfacesof the insulating material 20, the laterally constricted phase changematerial 8B, and the fill phase change material 24, as shown in FIG. 14.The top electrode 12 may be formed from one of the conductive materialspreviously described for use as the bottom electrode 10. The topelectrode 12 and underlying portions of the insulating material 20 maybe etched by conventional photolithography techniques to expose portionsof the dielectric material 13, forming the memory cells 4 shown inFIG. 1. By way of non-limiting example, a photoresist material (notshown) may be applied over the top electrode 12, and developed andetched according to conventional photolithographic techniques. If thebottom electrode 10 was formed as a layer, the bottom electrode 10, thetop electrode 12, and the underlying portions of the insulating material20 may be etched in situ by conventional photolithography techniques toform the memory cells 4. The adjacent memory cells 4 may be separated byan air gap 26. Alternatively, the air gap 26 may be filled with aninsulative material (not shown), such as a silicon oxide, a siliconnitride, or a material having a low thermal conductivity.

In the memory device 6 of FIG. 1, the laterally constricted portions 14of the laterally constricted phase change material 8B and the fill phasechange material 24 are in direct contact with the top electrode 12 andthe bottom electrode 10, respectively. The laterally constrictedportions 14 correspond to the programmable volumes 2 of the memory cells4. The laterally constricted phase change material 8B may have a shapethat is a reverse image of the shape of the fill phase change material24. Since the laterally constricted phase change material 8B and thefill phase change material 24 have inverted taper shapes of opposingorientations, the bit-to-bit distance d between the programmable volumes2 of adjacent memory cells 4 is maximized.

To form the memory device 6′ of FIG. 2, the mask material 16, the phasechange material 8, the bottom electrode 10, and the dielectric material13 may be formed as shown in FIG. 7. These materials may be formed aspreviously described in regard to FIG. 7. The mask material 16 and thephase change material 8 may be patterned, as shown in FIG. 15.Patterning of the mask material 16 and the phase change material 8 maybe conducted as previously described in regard to FIG. 8, except that aportion of the phase change material 8C remains over a top surface ofthe bottom electrode 10 and the dielectric material 13 in addition tothe patterned phase change material 8A. As such, the bottom electrode 10and the dielectric material 13 are not exposed. The partial removal ofthe phase change material 8 may be accomplished by conventional etchingtechniques, which are not described in detail herein. Patterning of themask material 16 and phase change material 8 produces openings 28, whichare defined by a bottom surface of the phase change material 8C andsubstantially vertical sidewalls of the patterned phase change material8A. The patterned mask material 16A may be trimmed, as shown in FIG. 16.Trimming of the mask material 16 may be accomplished as previouslydescribed in regard to FIG. 9. The trimmed mask material 16B may be usedas an etch mask to anisotropically etch the patterned phase changematerial 8A, producing laterally constricted phase change material 8B,which corresponds to phase change material element 8B, as shown in FIG.17. The first end 19 of the laterally constricted phase change material8B may have a decreased width compared to the second end 21 of thelaterally constricted phase change material 8B. The anisotropic etch ofpatterned phase change material 8A may be conducted as previouslydescribed in regard to FIG. 10. The phase change material 8C may besubstantially unaffected by the anisotropic etch.

The trimmed mask material 16B may be removed and the insulating material20 formed conformally over the exposed surfaces of the laterallyconstricted phase change material 8B and phase change material 8C, asshown in FIG. 18. The formation of the insulating material 20 may beconducted as previously described in regard to FIG. 11. Horizontalportions of the insulating material 20, such as those disposed on thetop surface of the laterally constricted phase change material 8B andphase change material 8C, may be removed, forming spaces 31 defined bythe substantially non-vertical or sloping sidewalls of the insulatingmaterial 20 and the top surface of the phase change material 8C, asshown in FIG. 19. The removal of the horizontal portions of theinsulating material 20 may be conducted as previously described inregard to FIG. 12. The spaces 31 may be filled with the fill phasechange material 24, as shown in FIG. 20. Filling the spaces 31 may beconducted as previously described in regard to FIG. 13. Another phasechange material 32 may be formed over the exposed surfaces of thelaterally constricted phase change material 8B, the insulating material20, and the fill phase change material 24, as shown in FIG. 21. Theanother phase change material 32 may be one of the materials previouslydescribed and may be the same or different than the phase changematerial 8 or fill phase change material 24. The another phase changematerial 32 may form a substantially planar layer over the laterallyconstricted phase change material 8B, the insulating material 20, andthe fill phase change material 24.

The top electrode 12 may then be formed overlying the another phasechange material 32. The top electrode 12 may be formed as previouslydescribed in regard to FIG. 14. As shown in FIG. 2, the top electrode 12and underlying portions of the insulating material 20, the another phasechange material 32, and the phase change material 8C may be etched byconventional techniques to expose portions of the dielectric material 13and form the memory cells 4. The adjacent memory cells 4 may beseparated by an air gap 26. Alternatively, the air gap 26 may be filledwith an insulative material (not shown), such as silicon oxide, asilicon nitride, or a material having a low thermal conductivity.

In the memory device 6′ of FIG. 2, the laterally constricted portions 14of the laterally constricted phase change material 8B and the fill phasechange material 24 are in direct contact with the phase change material8C and the another phase change material 32, rather than directlycontacting the bottom electrode 10 and the top electrode 12. The phasechange material 8C and the another phase change material 32 are insubstantial contact with the bottom electrode 10 and the top electrode12, respectively. The laterally constricted portions 14 correspond tothe programmable volumes 2 of the memory cells 4. The fill phase changematerial 24 may have a shape that is a reverse image of the shape of thelaterally constricted phase change material 8B. Since the laterallyconstricted phase change material 8B and the fill phase change material24 have alternating, inverted taper shapes of opposing orientations, thebit-to-bit distance d between the programmable volumes 2 of adjacentmemory cells 4 is maximized.

The memory devices 6, 6′ of FIGS. 1 and 2 may be fabricated using twophotomask processes. In one embodiment, a first photomask process isutilized to pattern the phase change material 8 and the mask material 16while the second photomask process is utilized to pattern the bottom andtop electrodes 10, 12 in situ. In a second embodiment, a first photomaskprocess is utilized to pattern at least a portion of the phase changematerial 8 and the mask material 16 while the second photomask processis utilized to pattern the bottom and top electrodes 10, 12 in situ.Since fabrication of a conventional memory device (in which theprogrammable volumes of adjacent memory cells are on the same verticalplane as one another) also utilizes two photomask processes, onephotomask process to pattern the bottom electrode and the otherphotomask process to pattern the memory cell and the top electrode,fabrication of the memory devices 6, 6′ may be accomplished withoututilizing additional process acts.

To form the memory device 6″ of FIG. 3, plugs of the bottom electrode 10may be formed in the dielectric material 13 as previously described inregard to FIG. 7. Additional dielectric material 34 may then be formedover the dielectric material 13 and the bottom electrode 10, andalternating wide apertures 36 and partially constricted wide apertures38 formed therein, as shown in FIG. 22. The wide apertures 36 andpartially constricted wide apertures 38 may be formed over the bottomelectrode 10. The wide apertures 36 and partially constricted wideapertures 38 may be formed using conventional photolithographytechniques followed by a dry etch process. By way of non-limitingexample, a mask (not shown) may be used to produce the wide apertures 36and narrow apertures 40 (shown in dashed lines). A portion of the narrowapertures 40 may be subsequently widened to form the partiallyconstricted wide apertures 38. The width of the partially constrictedwide apertures 38 at their widest point may be substantially the same asthe width of the wide apertures 36. By way of non-limiting example, atop portion of the narrow apertures 40 may be widened to form a lateralshelf 42 while a width of a bottom portion of the alternating narrowapertures 40 may remain substantially unaffected while masking thepartially constricted wide apertures 38. The lateral shelf 42 mayprovide the laterally constricted portion 14 to the partiallyconstricted wide apertures 38.

Alternatively, a mask (not shown) may be used to produce narrowapertures 40 in the dielectric material 13 overlying alternating bottomelectrodes 10 by conventional photolithography techniques. The width ofall or a portion of the narrow apertures 40 may then be increased,producing the wide apertures 36 and the partially constricted wideapertures 38, respectively. To form the wide apertures 36, the width ofthe alternating narrow apertures 40 may be widened. The alternatingnarrow apertures 40 may be widened by masking (not shown) those narrowapertures 40 that are not to be widened. Alternatively, those narrowapertures 40 that are not to be widened may be filled with a sacrificialmaterial (not shown). The narrow apertures 40 that are to be widened maythen be subjected to an anisotropic etch while those narrow apertures 40that are not to be widened are protected by the mask or sacrificialmaterial. The mask or sacrificial material may subsequently be removedby conventional techniques. To form the partially constricted wideapertures 38, a top portion of the alternating narrow apertures 40 maybe widened to form the lateral shelf 42 while a width of a bottomportion of the alternating narrow apertures 40 may remain substantiallyunaffected while masking the partially constricted wide apertures 38.The lateral shelf 42 may provide the laterally constricted portion 14 tothe partially constricted wide apertures 38. The phase change material 8may be deposited in the wide apertures 36 and the partially constrictedwide apertures 38, as shown in FIG. 23. The phase change material 8 maybe one of the materials previously described and may be deposited in thewide apertures 36 and the partially constricted wide apertures 38 byconventional techniques. Portions of the phase change material 8overlying the dielectric material 13 may be removed, such as by CMP.

Alternating wide apertures 44 and narrow apertures 46 may then be formedas shown in FIG. 24. The wide apertures 44 may be formed over the phasechange material 8 having the laterally constricted portion 14 thereinwhile the narrow apertures 46 may be formed over the phase changematerial 8 lacking the laterally constricted portion 14. The wideapertures 44 and narrow apertures 46 may be formed by removing a portionof the phase change material 8. Alternatively, additional dielectricmaterial (not shown) may be formed over the dielectric material 13 andphase change material 8 and portions of the additional dielectricmaterial are removed to form the wide apertures 44 and narrow apertures46. The wide apertures 44 and narrow apertures 46 may then be filledwith the another phase change material 32, as shown in FIG. 3. Theanother phase change material 32 may be one of the materials previouslydescribed and may be the same or different than the phase changematerial 8. The conductive material may be formed over the phase changematerial 8 and the another phase change material 32 and etched to formthe top electrode 12, as shown in FIG. 3.

As shown in FIG. 3, phase change material 8 and the another phase changematerial 32 of the memory cells 4 of the memory device 6″ may include afirst portion 48 and a second portion 50, where the width of the secondportion 50 may be greater than that of the first portion 48, forming aso-called “Y-shape.” Adjacent memory cells 4 may have alternatingY-shapes. An interface of the first portion 48 and the second portion 50may produce the laterally constricted portion 14 of the phase changematerial 8. The first portion 48 and the second portion 50 correspond tophase change material element 8B. The laterally constricted portions 14correspond to the programmable volumes 2 of the memory cells 4. Thefirst portions 48 of adjacent memory cells 4 may be vertically staggeredor offset in an alternating manner relative to one another and may,respectively, be in direct contact with one of the bottom electrode 10and the top electrode 12. Since adjacent memory cells 4 havealternating, inverted Y-shapes, the bit-to-bit distance d between theprogrammable volumes 2 of adjacent memory cells 4 is maximized.

The laterally constricted portions 14 in memory device 6 ensure that thecross-sectional contact area of the laterally constricted phase changematerial 8B or fill phase change material 24 in contact with the bottomelectrode 10 or top electrode 12 is minimized, which reduces the amountof current utilized to electrically switch the programmable volume 2between the amorphous and crystalline states. In memory device 6″, thedecreased width of the first portion 48 of the phase change material 8relative to the second portion 50 also provides a decreasedcross-sectional contact area for contact with the bottom electrode 10 orthe top electrode 12. In memory devices 6′, 6″, by positioning thelaterally constricted portions 14 such that the laterally constrictedportions 14 do not directly contact the bottom or top electrodes 10, 12,heat loss between the programmable volume 2 and the bottom or topelectrodes 10, 12 is reduced. As such, an interface between theprogrammable volume 2 and the bottom or top electrodes 10, 12 remainscool during use and operation of the memory devices 6′, 6″, whichimproves the reliability thereof. In addition, the reduced transversecross-sectional area of laterally constricted portions 14 increases thecurrent density therethrough, reducing the amount of current utilized toelectrically switch the programmable volume 2. The alternating, taperedshapes (in memory devices 6, 6′) or alternating, Y-shapes (in memorydevice 6″) serve to maximize the bit-to-bit distance d between adjacentmemory cells 4 during use and operation of the memory devices 6, 6′, 6″,reducing heat transfer between adjacent memory cells 4 andconsequential, unwanted phase changes.

To maximize the bit-to-bit distance d between adjacent memory cells 4and minimize the contact area between the programmable volume 2 of thephase change material 8 and its associated one of the bottom and topelectrodes, the memory device 6′″ shown in FIG. 4 may be formed. Thebottom and top electrodes of a single memory cell 4 may have differentwidths, such as a wide bottom electrode 54 and a narrow top electrode58. The bottom and top electrodes of an adjacent memory cell 4 may bevertically staggered or offset in an alternating manner, such that eachmemory cell 4 has a narrow bottom electrode 52 and a wide top electrode60 while a laterally adjacent memory cell has a wide bottom electrode 54and a narrow top electrode 58, and vice versa.

To form the memory device 6′″, the dielectric material 13 may bepatterned to form alternating, laterally adjacent narrow apertures (notshown) and wide apertures (not shown) therein. The dielectric material13 may be one of the materials previously described. The narrowapertures and wide apertures may be filled with the conductive materialto form narrow bottom electrodes 52 and wide bottom electrodes 54, asshown in FIG. 25. The conductive material may be one of the materialspreviously described. Portions of the conductive material overlying topsurfaces of the dielectric material 13 and the bottom electrodes 52, 54may be removed, such as by CMP.

An additional dielectric material 34 may be applied over the bottomelectrodes 52, 54 and the dielectric material 13. The additionaldielectric material 34 may be the same as or different from thedielectric material 13. The additional dielectric material 34 may bepatterned, as known in the art, to form a plurality of apertures (notshown) of equal width in the additional dielectric material 34. Theseapertures may be filled with the phase change material, forming phasechange material element 8B as shown in FIG. 26. The phase changematerial may be one of the chalcogenide compounds previously described.An upper surface of the memory device may then be planarized using, forexample, CMP. Alternatively, the phase change material may be depositedas a layer (not shown) over the bottom electrodes 52, 54 and thedielectric material 13, and patterned to form phase change materialelement 8B. The additional dielectric material 34 may then be appliedand subjected to CMP, producing the structure shown in FIG. 26.

Another dielectric material 56 may be formed over the dielectricmaterial 13 and the additional dielectric material 34. The anotherdielectric material 56 may be the same as or different from thedielectric materials 13, 34. The another dielectric material 56 may bepatterned, as known in the art, to form a plurality of alternating,laterally adjacent narrow apertures (not shown) and wide apertures (notshown). The narrow apertures in the another dielectric material 56 maybe formed over the memory cell locations having wide bottom electrodes54, while the wide apertures in the another dielectric material 56 maybe formed over the memory cell locations having narrow bottom electrodes52. The narrow apertures and wide apertures may be filled with theconductive material to form narrow top electrodes 58 and wide topelectrodes 60, as shown in FIG. 4. The conductive material may be one ofthe materials previously described and may be the same or different thanthe conductive material used for the bottom electrodes 52, 54. Portionsof the conductive material overlying top surfaces of the anotherdielectric material 56, the narrow top electrodes 58, and the wide topelectrodes 60 may be removed, such as by CMP.

Since the phase change material element 8B and the narrow electrodes(narrow top electrodes 58 or narrow bottom electrodes 52) have differentwidths, the surface area of the phase change material element 8B incontact with the narrow electrodes 52, 58 is minimized. As such, theamount of current utilized to electrically switch the programmablevolume 2 between the amorphous and crystalline states is reduced. Inaddition, by alternating the locations of the narrow electrodes (narrowtop electrode 58 or narrow bottom electrode 52), the bit-to-bit distanced between the programmable volumes 2 of adjacent memory cells 4 ismaximized.

While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, the invention is not limited to the particular forms disclosed.Rather, the invention encompasses all modifications, variations andalternatives falling within the scope of the invention as defined by thefollowing appended claims and their legal equivalents.

1. A method of forming a memory device, the method comprising: forming afirst electrode; forming a plurality of phase change material elementsover the first electrode, each phase change material element having aprogrammable volume, the programmable volumes of adjacent phase changematerial elements disposed at opposite ends of the phase change materialelements; and forming a second electrode over the plurality of phasechange material elements.
 2. The method of claim 1, wherein forming aplurality of phase change material elements over the first electrodecomprises forming a phase change material element having a first end anda second end, the first end having a width that is narrower than a widthof the second end.
 3. The method of claim 1, wherein forming a secondelectrode comprises forming a second electrode with a differenttransverse cross-section than a transverse cross-section of the firstelectrode.
 4. The method of claim 1, further comprising etching thefirst electrode or the second electrode in situ.
 5. The method of claim1, wherein forming a plurality of phase change material elements over afirst electrode comprises removing a portion of the phase changematerial elements to form laterally constricted portions thereof, eachlaterally constricted portion comprising the programmable volume of thephase change material element.
 6. The method of claim 1, wherein forminga plurality of phase change material elements over a first electrodecomprises applying a mask material over a phase change material andremoving a portion of the phase change material to form the plurality ofphase change material elements and a plurality of aperturestherebetween.
 7. The method of claim 6, further comprising selectivelytrimming the mask material.
 8. The method of claim 6, wherein formingthe plurality of phase change material elements and a plurality ofapertures therebetween comprises forming the plurality of phase changematerial elements having a nonuniform width.
 9. The method of claim 1,wherein forming a plurality of phase change material elements over afirst electrode comprises forming the plurality of phase change materialelements having alternating, inverted taper shapes of opposingorientations.
 10. The method of claim 1, further comprising formingadditional portions of horizontally positioned phase change material incontact with the programmable volumes of the phase change materialelements.
 11. The method of claim 1, further comprising forming an airgap between adjacent phase change material elements.
 12. A method offorming a memory device, the method comprising: forming a plurality ofphase change material elements over a first electrode, each phase changematerial element having a programmable volume, the programmable volumesof adjacent phase change material elements vertically staggered relativeto one another; and forming a second electrode over the plurality ofphase change material elements.
 13. The method of claim 12, whereinforming a plurality of phase change material elements over a firstelectrode comprises forming laterally constricted portions in each ofthe plurality of phase change material elements, each laterallyconstricted portion comprising the programmable volume of the phasechange material element.
 14. The method of claim 13, wherein forminglaterally constricted portions in each of the plurality of phase changematerial elements comprises forming each of the laterally constrictedportions at an interface of the phase change material elements and oneof the first electrode and the second electrode.
 15. The method of claim13, wherein forming laterally constricted portions in each of theplurality of phase change material elements comprises forming each ofthe laterally constricted portions within the phase change materialelements.